Comparison of antimicrobial susceptibility interpretation among Enterobacteriaceae using CLSI and EUCAST breakpoints

PurposeTo determine the difference in antimicrobial susceptibility of various antibiotics using the CLSI & EUCAST breakpoints.MethodsIn this non interventional, retrospective observational study, we reviewed minimum inhibitory concentrations (MIC) of various antibiotics routinely reported for Enterobacteriaceae clinical isolates, from an automated microbiology identification system (VITEK-2). These MICs were then analysed using both CLSI 2019 and EUCAST 2019 guidelines and classified as per the breakpoints into various categories.ResultsThe concordance rates of the antimicrobial susceptibility for various drugs ranged from 78.2% to 100% among two breakpoints. Perfect agreement with κ = 1 (p < 0.001) was observed for only three antimicrobials ceftriaxone, levofloxacin and trimethoprim-sulfamethoxazole. The changes in antimicrobial susceptibility interpretation for cefepime, ciprofloxacin, amoxicillin clavulanic acid was majorly in Intermediate category.ConclusionThe change in interpretation of the susceptibility will lead to change in the usage of antibiotics especially due to recent change in definition of I by EUCAST. There is need of more studies in this aspect to ascertain clinical implication of change in antimicrobial susceptibility.     

Review of antimicrobial resistance surveillance programmes in livestock and meat in EU with focus on humans

ObjectivesIn this review, we describe surveillance programmes reporting antimicrobial resistance (AMR) and resistance genes in bacterial isolates from livestock and meat and compare them with those relevant for human health.MethodsPublications on AMR in European countries were assessed. PubMed was reviewed and AMR monitoring programmes were identified from reports retrieved by Internet searches and by contacting national authorities in EU/European Economic Area (EEA) member states.ResultsThree types of systems were identified: EU programmes, industry-funded supranational programmes and national surveillance systems. The mandatory EU-financed programme has led to some harmonization in national monitoring and provides relevant information on AMR and extended-spectrum β-lactamase/AmpC– and carbapenemase-producing bacteria. At the national level, AMR surveillance systems in livestock apply heterogeneous sampling, testing and reporting modalities, resulting in results that cannot be compared. Most reports are not publicly available or are written in a local language. The industry-funded monitoring systems undertaken by the Centre Européen d’Etudes pour la Santé Animale (CEESA) examines AMR in bacteria in food-producing animals.ConclusionsCharacterization of AMR genes in livestock is applied heterogeneously among countries. Most antibiotics of human interest are included in animal surveillance, although results are difficult to compare as a result of lack of representativeness of animal samples. We suggest that EU/EEA countries provide better uniform AMR monitoring and reporting in livestock and link them better to surveillance systems in humans. Reducing the delay between data collection and publication is also important to allow prompt identification of new resistance patterns.     

Antimicrobial resistance 1979–2009 at Karolinska hospital,Sweden: normalized resistance interpretation during a 30‐year follow‐up on Staphylococcus aureus and Escherichia coli resistance development

Kronvall G. Antimicrobial resistance 1979–2009 at Karolinska hospital, Sweden: normalized resistance interpretation during a 30‐year follow‐up on Staphylococcus aureus and Escherichia coli resistance development. APMIS 2010; 118: 621–39. To utilize a material of inhibition zone diameter measurements from disc diffusion susceptibility tests between 1979 and 2009, an objective setting of epidemiological breakpoints was necessary because of methodological changes. Normalized resistance interpretation (NRI) met this need and was applied to zone diameter histograms for Staphylococcus aureus and Escherichia coli isolates. The results confirmed a slow resistance development as seen in Northern countries. The S. aureus resistance levels for erythromycin, clindamycin and fusidic acid in 2009 were 3.2%, 1.8% and 1.4% with denominator correction. A rise in resistance to four antimicrobials in 1983 was probably because of a spread of resistant Methicillin Susceptible Staphylococcus Aureus (MSSA). For E. coli, the denominator‐corrected resistance levels in 2009 were 27% for ampicillin, around 3% for third‐generation cephalosporins, 0.1% for imipenem, 2.5% for gentamicin, 19% for trimethoprim, 4.5% for co‐trimoxazole, 1.2% for nitrofurantoin and 9% for ciprofloxacin. The temporal trends showed a rise in fluoroquinolone resistance from 1993, a parallel increase in gentamicin resistance, a substantial increase in trimethoprim and sulphonamide resistance in spite of decreased consumption, and a steady rise in ampicillin resistance from a constant level before 1989. A short review of global resistance surveillance studies is included.     

Associated antimicrobial resistance in Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pneumoniae and Streptococcus pyogenes

Associated resistance to four to six related and unrelated antimicrobial agents was investigated in consecutive non-duplicate isolates of Escherichia coli (n = 39 425), Pseudomonas aeruginosa (n = 1070), Staphylococcus aureus (n = 7489), Streptococcus pneumoniae (n = 1604) and Streptococcus pyogenes (n = 2531). In all species, high proportions (76.5-88.9%) of isolates were susceptible to all the drugs investigated. Irrespective of species, isolates resistant to one drug were more likely to be resistant to any of the other drugs than were susceptible isolates. Thus, trimethoprim resistance in E. coli was 38.4% among ampicillin-resistant vs. 3.9% among ampicillin-susceptible isolates, and erythromycin resistance in Strep. pneumoniae was 41% among doxycycline-resistant vs. 1% among doxycycline-susceptible isolates. In all five species investigated, there was also significant associated resistance among unrelated drugs, highlighting the fact that resistance development occurs primarily among bacteria already resistant to one or more antimicrobial agents. For the clinician, pronounced resistance associations mean that when empirical therapy fails because of resistance, there is a reduced chance of choosing an alternative successful empirical agent. For the epidemiologist, who uses routine clinical susceptibility data to describe resistance development, resistance associations mean that if the dataset contains results for isolates selected on the basis of their susceptibility to another drug, structurally related or not, a bias of false resistance is introduced.     

Nationwide surveillance of antimicrobial resistance in invasive isolates of Streptococcus pneumoniae in Taiwan from 2017 to 2019

Background/purposeStreptococcus pneumoniae causes pneumonia and other invasive diseases, and is a leading cause of mortality in the elderly population. The present study aimed to provide current antimicrobial resistance and epidemiological profiles of S. pneumoniae infections in Taiwan.MethodsA total of 252 nonduplicate S. pneumoniae isolates were collected from patients admitted to 16 hospitals in Taiwan between January 2017 and December 2019, and were analyzed. The minimum inhibitory concentration of antibiotics was determined using the Vitek 2 automated system for antimicrobial susceptibility testing. Furthermore, epidemiological profiles of S. pneumoniae infections were analyzed.ResultsAmong the strains analyzed, 88% were recognized as invasive pneumococcal strains. According to the Clinical and Laboratory Standards Institute criteria for non-meningitis, the prevalence of penicillin-non-susceptible S. pneumoniae demonstrated a declining trend from 43.6% in 2017 to 17.2% in 2019. However, the rate of penicillin-non-susceptible S. pneumoniae was 85.7% based on the criteria for meningitis. Furthermore, the prevalence of ceftriaxone-non-susceptible S. pneumoniae was 62.7% based on the criteria for meningitis. Isolates demonstrated higher susceptibility toward doripenem and ertapenem than toward meropenem and imipenem. An increased rate of non-susceptibility toward levofloxacin was observed in southern Taiwan (15.1%) and elderly patients (≥65 years; 11.4%). Most isolates were susceptible to vancomycin and linezolid.ConclusionEmpirical treatment with ceftriaxone monotherapy for pneumococcal meningitis should be carefully monitored owing to its high non-susceptibility rate. The susceptibility rates of most isolates to penicillin (used for treating non-meningitis pneumococcal diseases), carbapenems (ertapenem and doripenem), respiratory quinolones (moxifloxacin and levofloxacin), vancomycin, and linezolid suggested the potential of these antibiotics in treating pneumococcal diseases in Taiwan.     

Escherichia coli clonal group A causing bacteraemia of urinary tract origin

Escherichia coli clonal group A (CgA) causes disease in humans. This is the first study investigating the prevalence of CgA among E. coli from non-urine, extraintestinal infections in a northern European country. E. coli blood (n = 196) and paired urine (n = 195) isolates from the same patients with bacteraemia of urinary tract origin were analysed. The isolates were collected from January 2003 through May 2005 at four hospitals in Copenhagen, Denmark. Pulsed-field gel electrophoresis (PFGE) patterns, antimicrobial resistance and patient characteristics were determined for all CgA isolates; presence of virulence-associated genes (VAGs) and serotypes were determined for the blood CgA isolates. Thirty blood isolates (15%) belonged to CgA. CgA blood isolates were associated with female patients and sulfamethoxazole-trimethoprim resistance and they harboured a distinctive VAG profile. The blood and urine isolates from each pair were found to be related in 26 of 27 CgA blood/urine pairs, confirming a urinary tract origin of infection. Furthermore, a relationship between the PFGE patterns of CgA blood/urine isolates and CgA isolates from UTI patients in general practice and a CgA isolate from a community-dwelling human reported previously, was found, suggesting a community origin of CgA. The finding of CgA strains in 15% of the E. coli bloodstream infections with a urinary tract origin in Denmark suggests that CgA constitutes an important clonal lineage among extraintestinal pathogenic E. coli. A reservoir of this pathogenic E. coli group in the community causing not only UTI but also more severe infections such as bacteraemia has implications for public health.     

Global epidemiology of antimicrobial resistance in commensal Neisseria species: A systematic review

BackgroundCommensal Neisseria species (spp). represent an important reservoir of antimicrobial resistance genes for pathogenic Neisseria spp. In this systematic review, we aimed to assess the antimicrobial susceptibility of commensal Neisseria spp. and how this has evolved over time. We also aimed to assess if commensal Neisseria spp. showed intrinsic resistance to four antimicrobials - penicillin, azithromycin, ceftriaxone and ciprofloxacin.MethodsPubmed and Google Scholar were searched following the PRISMA guidelines. Articles reporting MICs of commensal Neisseria spp. were included according to inclusion/exclusion criteria, and the quality of the articles was assessed using a pre-designed tool. Individual and summary measures of penicillin, azithromycin, ceftriaxone and ciprofloxacin MICs were collected. Additional data was sought to perform a comparison between the MICs of pathogenic and commensal Neisseria spp.ResultsA total of 15 studies met our criteria.We found no evidence of intrinsic AMR in commensal Neisseria spp. We did find evidence of an increasing trend in MICs of commensal Neisseria spp. over time for all antimicrobials assessed. These findings were similar in various countries. Eight additional studies were included to compare pathogenic and commensal Neisseria spp.ConclusionThe MICs of commensal Neisseria spp. appear to be increasing in multiple countries. Surveillance of MICs in commensals could be used as an early warning system for antimicrobial resistance emergence in pathogens. Our findings underline the need for antibiotic stewardship interventions, particularly in populations with high antimicrobial consumption.     

A prospective study to reduce turnaround time of microbiologically positive blood cultures in patients with sepsis in intensive care unit

PurposeTo determine the accuracy of direct microbial identification (dID) and antimicrobial susceptibility testing by EUCAST rapid antimicrobial susceptibility testing (rAST) methodology from positively flagged blood culture bottles (BCBs) as well as reduction in turnaround time (TAT) compared to standard methodology.MethodsIt was a hospital based, prospective cohort study conducted over a period of 21 months from March 2020 to November 2021 in which positively flagged blood culture bottles were simultaneously processed by dID ?+ ?rAST and by VITEK®-2 Compact system or Kirby-Bauer disk diffusion method. TAT was calculated as the time (hours) taken from receipt of sample in bacteriology laboratory to release of clinical reports with complete identification and susceptibility testing results in both methods.ResultsOf 301 dID ?+ ?rAST performed in study, 125 (41.5%) BCBs were identified as having one of the 8 reportable micro-organisms by EUCAST rAST standard. Amongst VITEK concordant BCBs with gram-negatives, mean reduction in TAT by dID ?+ ?rAST methodology was 23 ?± ?1.4 ?h. Amongst VITEK concordant gram-negatives, Categorical Agreement (CA) rates for any drug-bug combination were 94.4%, 94.5% and 93.6% and Very Major Error (VME) rates were 3.1%, 3.4% and 3.9% at 4-, 6- and 8-h reading time, respectively.ConclusionsEUCAST rAST methodology can generate susceptibility testing reports a day earlier if incorporated into the laboratory workflow. For resource-limited settings, implementing EUCAST rAST approach can be used effectively in early reporting, which can reduce antimicrobial use and improve patient outcomes by promoting timely escalation or de-escalation of empirical antibiotic therapy.     

The impact of multidrug resistance in healthcare-associated and nosocomial Gram-negative bacteraemia on mortality and length of stay: cohort study

Multidrug-resistant Gram-negative bacteria (MDR-GNB) are an emerging public health threat. Accurate estimates of their clinical impact are vital for justifying interventions directed towards preventing or managing infections caused by these pathogens. A retrospective observational cohort study was conducted between 1 January 2007 and 31 July 2009, involving subjects with healthcare-associated and nosocomial Gram-negative bacteraemia at two large Singaporean hospitals. Outcomes studied were mortality and length of stay post-onset of bacteraemia in survivors (LOS). There were 675 subjects (301 with MDR-GNB) matching study inclusion criteria. On multivariate analysis, multidrug resistance was not associated with 30-day mortality, but it was independently associated with longer LOS in survivors (coefficient, 0.34; 95% CI, 0.21–0.48; p < 0.001). The excess LOS attributable to multidrug resistance after adjustment for confounders was 6.1 days. Other independent risk factors for higher mortality included male gender, higher APACHE II score, higher Charlson comorbidity index, intensive care unit stay and presence of concomitant pneumonia. Concomitant urinary tract infection and admission to a surgical discipline were associated with lower risk of mortality. Appropriate empirical antibiotic therapy was neither associated with 30-day mortality nor LOS, although the study was not powered to assess this covariate adequately. Our study adds to existing evidence that multidrug resistance per se is not associated with higher mortality when effective antibiotics are used for definitive therapy. However, its association with longer hospitalization justifies the use of control efforts.     

Phylogenetic group,virulence factors and antimicrobial resistance of Escherichia coli associated with bovine mastitis

Escherichia coli is an important pathogen involved in the etiology of bovine mastitis. A total of 70 E. coli isolates recovered from clinical and subclinical mastitis samples were characterized with respect to their phylogenic group, virulence factors and antimicrobial susceptibility. Based on the presence of the specific genes chuA, yjaA and TspE4.C2, these isolates were found to belong to three different groups: group A(25), group B1(41) and group D(4). Twenty-five (35.7%) isolates harbored at least one virulence gene, and the most prevalent virulence genes were f17A, irp2, astA, iucD and colV. The irp2-coding gene was more often detected in group A than in group B1 isolates; in contrast, colV was identified more often in group B1 isolates. The majority of isolates (87.1%) were resistant to at least one antimicrobial compound. Forty-seven isolates (67.1%) were resistant to streptomycin, and those from group B1 were more resistant to streptomycin than isolates from group A. The latter feature was supported by the distribution of streptomycin resistance genes observed in group B1 compared to group A.     

Integrons and multidrug resistance among Escherichia coli causing community-acquired urinary tract infection in southern India

Antimicrobial resistance genes are often clustered in integrons, genetic elements capable of recombination. There is a paucity of data on the prevalence and role of integrons from community-acquired infections in developing countries where resistance to co-trimoxazole is high. We determined the prevalence of integrons among Escherichia coli causing community-acquired urinary tract infection (UTI). Consecutive isolates of E. coli obtained from UTI of pregnant women at the Christian Medical College Hospital, Vellore, India, during 2002 were included. All isolates were tested for susceptibility to 16 antimicrobials using the disc diffusion method and for integrons of classes 1 and 2 by PCR. Of the 58 isolates tested, 28 (48.3%) were resistant to co-trimoxazole and trimethoprim. All these isolates carried integrons. Three additional isolates were sulfonamide resistant but integron negative. Class 1 integrons were present in 21 (36.2%) isolates. Resistance to ampicillin (p=0.000), nalidixic acid (p=0.001), chloramphenicol (p=0.02), tetracycline (p=0.004) and gentamicin (p=0.02) was significantly more common in isolates with integrons. DNA sequencing of two isolates with integrons showed the presence of aadA, dfr1 and dfr7 genes. This study demonstrated that integrons are widely prevalent in India and that they might play a role in multidrug resistance in E. coli from community-acquired UTI.     

125例糖尿病足感染患者病原菌分布及耐药性分析

目的：分析糖尿病足部感染临床分离病原菌的分布及其抗菌药物敏感性特点,为临床医务工作者合理应用抗菌药物提供理论依据。方法：选取2011年1月至2014年9月,来本院治疗的糖尿病足合并感染患者125例,采集糖尿病足部溃疡分泌物,并保存培养分离出的病原菌,采用琼脂倍比稀释法进行药物敏感试验,药敏结果依据美国临床实验室标准化委员会2013年推荐的标准进行判读。结果：125例糖尿病足感染患者溃疡分泌物共培养分离出病原菌132株,其中,革兰阳性菌62株(47.0%),革兰阴性菌55株(41.7%),真菌15株(11.4%)。革兰阴性菌对碳青霉烯类抗菌药物和含β-内酰胺酶抑制剂的药物敏感性相对较高;革兰阳性菌对糖肽类抗菌药物、碳青霉烯类抗菌药物、氨基糖苷类药物的敏感性相对较高;真菌则对两性霉素B和卡泊芬净敏感度最高。结论：感染是糖尿病足患者病情加重的一个重要因素,对于糖尿病足部感染患者要尽早并多次进行分泌物的病原菌分离和药敏试验,以指导临床工作者筛选敏感的抗菌药物进行针对性治疗,减少耐药菌的产生。     

Investigation of Ureaplasma urealyticum biovars and their relationship with antimicrobial resistance

Purpose: To develop Taqman fluorescence quantitative polymerase chain reaction (PCR) method for investigating the characteristics of the distributions of Ureaplasma urealyticum (UU) biovars and to explore the relationship between UU biovars and antimicrobial resistance. Materials and Methods: By the method of culture, Ureaplasma species were detected. Taqman fluorescence quantitative PCR for detecting UU biovars were developed and UU clinical isolates were detected to distinguish biovars. The broth micro-dilution susceptibility testing methods were used to determine UU susceptibility. Results: By Taqman PCR method, UU biovars was successfully detected. Of 126 samples, biovar 1 was found in 73 (57.94%). There was a statistical difference between genital-urinary tract infection group and asymptomatic group (P<0.05). In the region, UU biovar 1 to 9 kinds of agents kept higher susceptibility rates, but biovar 2 maintained higher susceptibility rates only to tetracyclines. Compared with biovar 1, UU biovar 2 resistance rates to 7 kinds of agents were higher (P<0.05). Conclusions: (1) Our new established Taqman PCR method is a useful tool for screening UU biovars. (2) UU biovar 1 predominated in asymptomatic population; whereas in genital-urinary tract infection population UU biovar 2 had a higher proportion. (3) The characteristics of drug resistance were different between UU biovars. Overall, both two biovars remained higher susceptibility rates to tetracyclines. A majority of biovor 1 strains were sensitive to macrolides and quinolones; while only a small number of biovar 2 strains kept sensitive to roxithromycin and quinolones, a large proportion of biovar 2 strains were found in intermediate ranges.     

Surveillance of resistance in bacteria causing community-acquired respiratory tract infections

Bacterial resistance to antibiotics in community-acquired respiratory tract infections is a serious problem and is increasing in prevalence world-wide at an alarming rate. Streptococcus pneumoniae , one of the main organisms implicated in respiratory tract infections, has developed multiple resistance mechanisms to combat the effects of most commonly used classes of antibiotics, particularly the β -lactams (penicillin, aminopenicillins and cephalosporins) and macrolides. Furthermore, multidrug-resistant strains of S. pneumoniae have spread to all regions of the world, often via resistant genetic clones. A similar spread of resistance has been reported for other major respiratory tract pathogens, including Haemophilus influenzae , Moraxella catarrhalis and Streptococcus pyogenes . To develop and support resistance control strategies it is imperative to obtain accurate data on the prevalence, geographic distribution and antibiotic susceptibility of respiratory tract pathogens and how this relates to antibiotic prescribing patterns. In recent years, significant progress has been made in developing longitudinal national and international surveillance programs to monitor antibiotic resistance, such that the prevalence of resistance and underlying trends over time are now well documented for most parts of Europe, and many parts of Asia and the Americas. However, resistance surveillance data from parts of the developing world (regions of Central America, Africa, Asia and Central/Eastern Europe) remain poor. The quantity and quality of surveillance data is very heterogeneous; thus there is a clear need to standardize or validate the data collection, analysis and interpretative criteria used across studies. If disseminated effectively these data can be used to guide empiric antibiotic therapy, and to support—and monitor the impact of—interventions on antibiotic resistance.     

Bacterial profile and their antimicrobial susceptibility patterns in patients admitted at MaddaWalabu University Goba Referral Hospital,Ethiopia: a cross sectional study

BackgroundHospital acquired infections (HAIs) are one of the global concerns in resource limited settings. The aim of the study was to determine bacteria profile and their antimicrobial susceptibility patterns among patients admitted at surgical and medical wards.MethodsA hospital based cross-sectional study was conducted from November 2016 to July 2017 in MaddaWalabu University Goba Referral Hospital. Urine and wound swabs were processed and standard disk diffusion test was done to assess susceptibility pattern. Association among variables was determined by Chi-square test.ResultsAmong 207 patients enrolled, 24.6% developed HAI, of which, 62.7% and 37.3% were from surgical and medical wards, respectively. The male to female ratio was 1.5:1. The age ranged from 19 to 74 years with a mean of 41.65(±16.48) years. A total 62 bacteria were isolated in which majority of the isolates were gram negative bacteria. Most isolates were resistance to most of the antibiotics tested but sensitive to Ceftriaxone, Norfloxacin and Ciprofloxacin.ConclusionDue to the presence of high level drug resistant bacteria, empirical treatment to HAI may not be effective. Therefore, treatment should be based on the result of culture and sensitivity.     

Molecular epidemiology of Escherichia coli causing bloodstream infections in a centralized Canadian region: a population-based surveillance study

ObjectivesEscherichia coli is a leading cause of bloodstream infections worldwide, and is responsible for substantial patient morbidity, mortality and healthcare expenditure. Understanding the molecular epidemiology of E. coli will aid in designing superior treatment and prevention strategies.MethodsWe undertook a population-based surveillance study describing the clinical factors, susceptibility patterns, incidence rates and geographical distribution of sequence types (STs) among E. coli isolates (n = 686) causing incident bloodstream infections in a centralized Canadian region during 2016. STs were identified using a seven-single-nucleotide-polymorphism quantitative PCR (n = 422) and sequencing of certain house-keeping genes (n = 249).ResultsThe annual population incidence rate of E. coli bloodstream infections was 48.8/100 000 patient years, and five dominant clones (ST131, ST73, ST69, ST95 and ST1193) accounting for 55% (378/686) of the population were identified, each with a specific geographical distribution within Calgary. ST131 was the most common (overall incidence rate of 10.4/100 000 patient years), an antimicrobial-resistant (AMR) clone affecting mainly the elderly and the very young. ST131 was common among residents in long-term care with an incidence rate of 312.5/100 000 patient years. ST73 was associated with community infections in the elderly, while ST69 and ST95 had increased incidence rates among females. ST1193 was the second most AMR clone and was associated with bloodstream infections in elderly males.ConclusionsThis study showed that E. coli clones have unique characteristics in a well-defined human population. The elimination of ST131 would substantially decrease the overall incidence rate and AMR burden among E. coli bloodstream infections in the Calgary region, leading to considerable public health benefits.     

The critical influence of the intermediate category on interpretation errors in revised EUCAST and CLSI antimicrobial susceptibility testing guidelines

Erroneous assignments of clinical isolates to the interpretative categories susceptible, intermediate and resistant can deprive a patient of successful antimicrobial therapy. The rate of major errors (ME) and very major errors (vME) is dependent on: (i) the precision/standard deviation (σ) of the antibiotic susceptibility testing (AST) method, (ii) the diameter distributions, (iii) clinical breakpoints, and (iv) the width of the intermediate zone. The European Committee on AST (EUCAST) has abandoned or decreased the intermediate zone for several drug/species combinations. This study focused on the effects of discontinuing the intermediate category on the rate of interpretation errors. In total, 10 341 non-duplicate clinical isolates were included in the study. For susceptibility testing the disc diffusion method was used. Error probabilities were calculated separately for diameter values flanking the interpretative category borders. Error probabilities were then applied to the actual numbers of clinical isolates investigated and expected rates of ME and vME were calculated. Applying EUCAST AST guidelines, significant rates of ME/vME were demonstrated for all drug/species combinations without an intermediate range. Virtually all ME/vME expected were eliminated in CLSI guidelines that retained an intermediate zone. If wild-type and resistant isolates are not clearly separated in susceptibility distributions, the retaining of an intermediate zone will decrease the number of ME and vME. An intermediate zone of 2–3 mm avoids almost all ME/vME for most species/drug combinations depending on diameter distributions. Laboratories should know their epidemiology settings to be able to detect problems of individual species/drug/clinical breakpoint combinations and take measures to improve precision of diameter measurements.